Nitrogenated ultrananocrystalline diamond/hydrogenated amorphous carbon composite films were prepared in a hydrogen and nitrogen mixed-gas atmosphere by pulsed laser deposition with graphite targets. The electrical conductivity in n-type conduction remarkably increases to 18 Ω-1-cm-1 at 300 K with an increase in the nitrogen content up to 7.9 at.%. In the nitrogen content range from 7.9 to 1i0.4 at.%, the electrical conductivity is suddenly and dramatically decreased down to 0.03 Ω-1 cm-1 in reverse, accompanied by the disappearance of diamond grains in the films. A huge number of grain boundaries owing to the existence of diamond grains embedded in UNCD/a-C:H films, which is structural specific to UNCD/a-C:H, should play a significant role in the large electrical conductivity enhancement by nitrogen doping. The X-ray photoemission and near-edge X-ray fine-absorption spectroscopic measurements could not detect an evident difference in the spectra that explain the sudden irregular change in the electrical conductivity.